When storing cells, tissue or other biological material, there is always the danger of contamination from bacteria, viruses, yeasts, molds, fungi etc., and sometimes the contaminants are present in the biological material when it is first collected. Contaminants are such agents that may damage the biological material during preservation and/or harm the recipient when the product is used (e.g. transfused, injected or eaten). Among known contaminants are white blood cells (WBC) that are normally present in red blood cell (RBC) samples. The presence of WBC in a transfusion liquid is a problem due to graft vs. host disease, in which the transfused WBC (mainly the lymphocytes) attack the recipient's body.
Many methods for sterilization are known in the art including heating and filtration. However, these processes may damage biological material (e.g. when it is sensitive to heat) or prove to be inefficient (e.g. when the biological material is filtered with some contaminants). Other ways for sterilization involve ionizing radiation, mainly gamma rays. For example, gamma radiation is used for inactivation of WBC, mainly the lymphocytes which are the main cause for graft versus host disease (GVHD), in fresh blood units. This is normally done by irradiating a liquid sample of blood or blood components including RBC, platelets, granulocyte components and non frozen plasma in a plastic bag with 2.5 mega Rad of gamma radiation to the central portion of the bag, resulting with no less than 1.5 mega Rad which are delivered to every part of the blood bag (AABB Technical Manual, 14th edition). Attempts were made to reduce the bacteria content in platelet concentrates (Hustom et al. 1998) but it was concluded that exposure to gamma radiation at levels up to 7.5 mega Rad was ineffective at sterilizing the sample. Likewise it was found that gamma radiation (1.5-2.5 mega Rad) does not constitute a virucidal dose for HIV type 1 in frozen bone and tendon allografts (Smith et al. 2001).
Furthermore, gamma radiation can be damaging to radiation-sensitive products. In particular it has been shown that gamma radiation is injurious to red blood cells, platelets and granulocytes (US 2004/067157).
Ultraviolet (UV) radiation on the other hand is considered less damaging than gamma radiation. However, as UV radiation is absorbed by water, it is practically ineffective for removal of contaminants that are in a water-containing sample (liquid or ice). Accordingly it was suggested in WO 2004/0091938 that reduction of the residual solvent content of biological material would reduce the absorption of UV in the water and thus enable sterilization of a biological sample using V. However, sterilization of biological material in WO 2004/0091938 was restricted to wet biological material or to non-cellular portions of a blood preparation (i.e. not including RBC or platelets), apparently since “sensitive biologicals, such as blood, would lose viability and activity if subjected to freezing for irradiation purposes and then thawing prior to administration to a patient” (id.).